To provide a high quality cut at the desired cutting height, mower decks having a pair of rotary cutting blades have been designed to lift the grass under the mower deck. Mower blades may be shaped to lift the grass as they rotate at high speed. The blades may be enclosed within depending cylindrical cutting chambers having skirts or perimeter walls that are configured to contain the air stream generated by the rotating blades and assist in lifting the grass to cut it at the desired height. Discharge chutes may discharge the grass clippings from the cutting chambers into a collection container. A sufficiently strong air stream is needed to lift the grass to the desired cutting height, and discharge the grass out the rear of the mower deck into the collection container.
To lift heavy grass clippings, and avoid or reduce clogging, blowers or fans may be mounted on the mower deck or in a discharge chute to the collection container. However, supplemental air handling devices have disadvantages including power requirements, noise, and cost. As a result, it often is preferable to discharge grass clippings from the mower deck to a collection container using only air pressure provided by the cutting blades and surfaces under the deck.
Mower deck cutting chambers have downwardly extending perimeter skirts or baffles which extend below the horizontal plane of the rotary cutting blades during mowing. The skirts or baffles may be intended to optimize air flow in each cutting chamber, as well as between the cutting chambers and/or through a rear discharge opening. Additionally, some mower decks may be designed to provide an air flow into the cutting chambers. For example, U.S. Pat. No. 6,681,553 entitled Mowing Cutting Chamber relates to a cutting chamber for a mower deck blade having skirt members of differing lengths to provide for a lifting air stream at the front of the chamber and a discharging air stream at the rear of the chamber. U.S. Pat. No. 3,795,095 entitled Vented Rotary Lawn Mower relates to a mower deck having a plurality of air vents which permit air to enter the top of the deck and pass downwardly to assist air flow.
At lower cutting heights, the perimeter skirts or baffles around the cutting chambers under a mower deck may be relatively close to the ground. Their proximity to the ground can reduce and restrict the flow of air into the cutting chambers. Reduced and restricted air flow into the cutting chambers can reduce the lift needed to cut grass at a desired height, and can reduce the air flow needed to propel the clippings into the collection container. As a result, fewer grass clippings may be discharged from the mower deck into the container.
Additionally, air flow from each rotary cutting blade may blow over grass near the center of the mower deck. As a result, the pair of blades may not cut the grass at or near the middle of the deck to the desired height, especially at lower cutting heights. To avoid leaving a strip or swath of longer or uncut grass at the middle of the deck, some mower decks may position one of the cutting blades forward of the other(s). Other mower decks may use timing mechanisms so that the blades can overlap. For example, U.S. Pat. No. 6,065,276 entitled Mulching Mower Deck relates to a mower deck with overlapping blades and a flow divider extending down from the top wall, positioned generally above the location where the blade paths overlap.
A mower deck is needed that can provide rear discharge of grass clippings into a collection container without a supplemental fan or blower. A mower deck is needed that can avoid leaving a swath or strip of long or uncut grass in the middle of the deck between the cutting blades, with minimal cost and complexity. A mower deck is needed that can mow and collect grass at lower cut heights.